The present invention relates to a method for producing printed wiring boards (PWBs), especially double-side PWBs or multilayer PWBs.
Recently, a low cost multilayer PWB that enables high-density mounting of semiconductor chips such as LSIs has been desired for industrial machines as well as home electronic appliances. It is important for such multilayer PWBs to provide high quality electric connections between plural layers of wiring patterns formed at a fine spacing or pitch.
The conventional PWB that is produced by drilling, etching and plating a copper-foil laminated board cannot satisfy the above-mentioned need anymore for sophisticated electronic equipment having a number of functions. To solve such a problem, some methods are under development for producing PWBs that have a new structure and a high density of wiring.
One of the methods is a recent technique for forming a fine pattern that can be applied to a high-density surface mounting. This method for producing PWBs utilizes a plating technique and a transferring technique for the wiring pattern. Two metal sheets are prepared, each of which has a surface with a wiring pattern formed by electroplating of copper. A semi-hardened resin sheet such as a prepreg is placed between the wiring patterns of the metal sheets. Heat and pressure are applied to the outer surfaces of the metal sheets. Thus, the copper wiring patterns are transferred from the surfaces of the metal sheets to the surfaces of the resin sheet. After removing the metal sheets, through holes are formed by drilling in the resin sheet, and copper plating is performed inside the through holes to connect the wiring pattern on one side with that of the other side electrically (Naoki Hukutomi et al. xe2x80x9cDevelopment of Fine Pattern Wiring Techniquexe2x80x9d, The Institute of Electronics, Information and Communication Engineers, C-II, Vol. J72-C-II, No. 4, PP243-253, 1989). This method provides a line width and a line space of 20 microns each.
There is another technique called xe2x80x9cALIVHxe2x80x9d (a trademark of Matsushita Electric Industrial Co., Ltd.), that is a resin-based multilayer PWB having an inner via hole (IVH) structure for all layers. In this multilayer PWB, a conductive material is filled in the inner via holes instead of copper plating inside the through holes that is a major method for electric connection between layers of a multilayer PWB in the prior art. This xe2x80x9cALIVHxe2x80x9d PWB thus improves the reliability of the electric connection between layers, and facilitates forming inner via holes under lands for mounting components or between any layers (U.S. Pat. Nos. 5,346,750 and 5,481,795).
An example of the method for producing the xe2x80x9cALIVHxe2x80x9d PWB is explained below, referring to FIGS. 5A-5F that show cross sections in the producing process. As shown in FIG. 5A, via holes 502 are perforated by using a laser beam machine at predetermined positions in an adhesive insulator sheet 501 that comprises an aramid-epoxy prepreg made of a non-woven aramid sheet impregnated with an epoxy resin. Then, as shown in FIG. 5B, the via holes 502 are filled with a fluid conductive paste 503. Then, as shown in FIG. 5C, the adhesive insulator sheet 501 with via holes filled with the conductive paste is placed between copper foils 504, and heat and pressure are applied to the outer surfaces of the copper foils. Thus, the adhesive insulator sheet (the prepreg) 501 and the conductive paste 503 are hardened, the copper foils 504 adhere to the surfaces of the adhesive insulator sheet 501, and electrical connections are formed between the copper foils by the conductive paste 503 packed into the via holes 502. The copper foils 504 are etched by a conventional photolithography method to form wiring patterns 505a, 505b. Thus, a double-side PWB 506 is obtained as shown in FIG. 5D.
In the next step shown in FIG. 5E, the double-side PWB 506 is used as a core, and on both sides of the core PWB 506 other adhesive insulator sheets 501a and 501b are placed with proper registration. These adhesive insulator sheets 501a, 501b have been made previously according to the step shown in FIG. 5B, and each of them has via holes filled with the conductive paste at predetermined positions. On the outer surfaces of the adhesive insulator sheets 501a and 501b, copper foils 507a and 507b are placed. Heat and pressure are applied to both outer surfaces of the copper foils 507a, 507b for lamination. Then, similarly to the step of FIG. 5D, the outer copper foils 507a, 507b are etched by the photolithography method. Thus a four-layer PWB is obtained having the outer wiring patterns 508a, 508b as shown in FIG. 5F. This method for producing PWBs enables via-connections (electric connections between layers) with very small via holes since via holes are formed by a laser beam and filled with the fluid conductive paste for the electric connection.
However, in the above-mentioned transferring technique of the wiring pattern, there is a limit for reducing the size of through holes since they are perforated by machining. On the other hand, in the above xe2x80x9cALIVHxe2x80x9d PWB, there is a limit on the fineness of patterns with respect to pattern density such as a line pitch and a line width since the outer and inner copper patterns are formed by the conventional photolithography method. These limits are obstacles to producing high density mounting of surface mount components, especially small electronic components such as recent chip components or LSI bare chips.
The present invention provides a method for producing fine pattern PWBs that enables high density mounting of components by combining the advantage of the conventional transferring method of wiring patterns and the advantage of the conventional xe2x80x9cALIVHxe2x80x9d structured multilayer PWB.
The method for producing PWBs according to the present invention comprises the steps of perforating through holes at predetermined positions in an adhesive insulator sheet, filling the through holes with a conductive material, forming conductive wiring patterns on surfaces of releasable supporting sheets, and transferring the conductive wiring patterns from the surface of the releasable supporting sheets onto surfaces of the adhesive insulator sheet so as to form the wiring patterns on the surfaces of the adhesive insulator and perform electric connections between the wiring patterns of plural layers.
According to the present invention, PWBs can be produced with fine patterns defining very fine wiring pitches and electric connections by very small via holes. In addition, such fine PWBs can be produced at a low cost since the method according to the present invention is simple compared with other conventional methods.
Another method according to the present invention uses the PWB produced by the method mentioned above as a core. Second adhesive insulator sheets are prepared that have through holes filled with a conductive material. These second adhesive insulator sheets are placed on surfaces of the core PWB. On the outer surfaces of the second adhesive insulator sheets, second releasable supporting sheets are placed, whose surfaces facing the second adhesive insulator sheets are provided with second conductive wiring patterns. The second conductive wiring patterns are transferred from the surfaces of the second releasable supporting sheets onto the surfaces of the second adhesive insulator sheets so as to form surface wiring patterns and perform electric connection between the surface wiring patterns and the inner wiring patterns. Thus, multilayer PWBs with fine patterns can be produced inexpensively. By repeating the steps mentioned above, PWBs having more layers can be produced easily.
Another method according to the present invention uses a double-side or multilayer PWB produced by a conventional method as a core. Adhesive insulator sheets that have via holes filled with a conductive material are placed on surfaces of the core PWB. Conductive wiring patterns on surfaces of releasable supporting sheets are transferred onto outer surfaces of the adhesive insulator sheets so as to form surface wiring patterns and perform electric connection between the surface wiring patterns and the inner wiring patterns. Also in this method, PWBs having more layers can be produced easily by repeating the steps mentioned above.
In each method mentioned above, it is preferable to use a fluid conductive paste as the conductive material to be packed into the through holes. The use of the fluid conductive paste enables reliable electric connections with very small via holes.
It is preferable that conductive sheets are used as the releasable supporting sheets and the conductive wiring patterns are formed by electroplating after forming resist films on surfaces of the releasable supporting sheets. Finer patterns can be obtained by electroplating after printing fine resist pattern than etching conductive layer. Furthermore, production cost may be reduced because of less waste of conductive material.
Alternatively, the conductive wiring pattern can be formed by printing a conductive paste on the surface of the releasable sheet. This method enables formation of conductive wiring patterns at a low cost.
It is also preferable that insulated and via-connected multilayer wiring patterns are formed in the step of forming wiring patterns on the surface of releasable supporting sheets. This produces PWBs having plural inner wiring patterns at a single transferring.
It is preferable to use a semi-hardened resin insulator as the adhesive insulator sheet. This semi-hardened insulator sheet becomes hardened completely when the heat and pressure are applied for transferring the wiring pattern from the surface of the releasable supporting sheet onto the surface of the adhesive insulator sheet. Thus, a large adhesion strength is obtained between the conductive wiring pattern and the hardened insulator sheet.
The adhesive insulator sheet is preferably made of a porous and compressible semi-hardened material. This adhesive insulator sheet is compressed when the heat and pressure are applied for transferring the wiring pattern from the surface of the releasable supporting sheet onto the surface of the adhesive insulator sheet. Simultaneously, the conductive material in the via holes is compressed so that reliable via-connections with good conductivity can be obtained.
More preferably, the adhesive insulator sheet is made of a prepreg that is a non-woven aramid sheet impregnated with an epoxy resin. This material has ideal properties of transferring ability and compressibility. Since this material has light weight and its thermal expansivity is as low as a ceramic PWB, PWBs of high utility with low dielectric constant and high resistance to heat are obtained.
In each method for producing PWBs mentioned above, it is preferable to use heat and pressure for transferring the conductive pattern from the surface of the releasable supporting sheet onto the surface of the adhesive insulator sheet and hardening the adhesive insulator sheet completely. The heat and pressure make the conductive wiring pattern transfer and adhere securely to the surface of the adhesive insulator sheet. Thus, the PWB produced by this method has a fine wiring pattern and high mechanical strength due to the promoted cure of the adhesive insulator.
The PWB according to the present invention comprises an adhesive insulator sheet having through holes filled with a conductive material, a conductive wiring pattern that is formed on the surface of the adhesive insulator sheet by the transferring method and is connected with the conductive material in the via holes, wherein the conductive wiring pattern is embedded in the adhesive insulator sheet so as to form a flat surface. Such a PWB is suitable for flip mounting of LSI chips since the surface of the PWB is flat.
It is preferable that a width of the conductive wiring pattern formed on the adhesive insulator sheet is smaller than a diameter of the via hole at least where the conductive wiring pattern is overlaid on the via hole. This configuration can relieve a maximum allowable limit of pattern deviation, which is critical for PWBs with fine patterns.